I. Field of the Invention
The present invention relates to Code Division Multiple Access (CDMA) systems. More particularly, the present invention includes, but is not limited to, a novel and improved CDMA base station that shapes the frequency spectrum of CDMA signals to reduce out-of-band signal emissions.
II. Description of the Related Art
Code Division Multiple Access (CDMA) technology is commonly used in communications systems. In a typical CDMA system, a CDMA base station transmits a CDMA signal to numerous CDMA communications devices, such as wireless telephones. The CDMA signal is comprised of numerous individual user signals. The CDMA base station generates the CDMA signal by encoding each individual user signal with a unique spreading sequence, such as a pseudo random sequence. The CDMA base station then adds the encoded user signals together to form the CDMA signal.
In a CDMA system, individual user signals are not separated based on frequency or time, but are spread across the entire frequency band. Each CDMA communications device derives its particular user signal based on the unique spreading sequence. Due to this combination of multiple signals encoded with random sequences, the CDMA signal has random signal peaks that cause problems when the CDMA signal is amplified.
The CDMA base station uses a power amplifier to amplify the CDMA signal. The power amplifier contributes unwanted noise when operated above a maximum power level. Unfortunately, the random peaks in the CDMA signal force the power amplifier to operate above this maximum power level. In contrast, the typical Frequency Modulated (FM) signal does not have random signal peaks, so the power amplifier is able to continuously operate below the maximum power level.
The power amplifier contributes noise in the form of signal power outside of the frequency band of the CDMA signal. This signal power is referred to as out-of-band signal power. Out-of-band signal power is a problem because it interferes with other signals in the neighboring frequency bands. These other signals are disrupted by the interference. Government agencies, such as the Federal Communications Commission in the United States, strictly regulate the interference caused by out-of-band signal power.
The existing solution to this problem is to operate the power amplifier in the CDMA base station below its maximum power level. This reduces the amount of out-of-band signal power caused by the random peaks in the CDMA signal. This solution is lacking because the power and range of the base station is reduced. In addition, the power amplifier may operate less efficiently below the maximum power level.
CDMA systems would be improved by techniques to reduce the noise contribution of the power amplifier in the base station. The noise reduction would directly increase the power and efficiency of the CDMA base station.
The above-described problem is solved with CDMA spectral shaping technology that reduces the out-of-band signal power in the CDMA signal. The in-band CDMA signal is attenuated near the corner frequencies to reduce components that provide a disproportionate contribution to the out-of-band signal power. The power amplifier in the CDMA base station can then operate at higher power levels without exceeding out-of-band signal power limitations. As a result, the power amplifier operates more efficiently and extends the range or capacity of the base station. This improvement is passed on to the wireless communications user in the form of higher quality and lower cost.
In some examples of the invention, the corner filters are digital elements that are placed between the cell site modem and the digital-to-analog converter in the base station. In other examples of the invention, the corner filters are analog elements that are placed between the digital-to-analog converter and the low-pass filter in the base station.